1. Field of the Invention
This invention relates to a process for simultaneously etching holes in both thick and thin portions of a dielectric layer on a semiconductor substrate. More particularly, the process is used to eliminate a mask-and-etch cycle in the manufacturing of integrated circuits.
2. Description of the Related Art
The etching of dielectric layers on a semiconductor substrate is critical to the manufacture of integrated circuits. Repeated masking and etching cycles of the dielectric layers are used to define the minute structural patterns of the integrated circuit devices. Wet etching processes are the conventional etching processes used. These processes etch isotropically. Therefore, significant lateral, as well as vertical, etching occurs during processing. The conventional wet etching of a dielectric layer 12 on a semiconductor substrate 11 is illustrated in FIGS. 1 and 2. The area of dielectric layer 12 to be etched is defined by a window 14 in a conventional photolithographic mask 13. Upon exposure of dielectric layer 12 to wet etching, an area 16 of semiconductor substrate 11 is exposed. Vertical etching is represented by arrow 17 and lateral etching by arrow 18 of FIG. 2. Thus, the size of area 16 is a function of the size of window 14, the amount of vertical etching, and the amount of lateral etching that occurs in dielectric layer 12. Wet etching is required for a long enough time to ensure complete vertical etching 17 of dielectric layer 12, yet the etching time must be minimized to avoid excessive lateral etching 18. Excessive lateral etching results in the increased size of area 16, thereby making maintenance of the dimensional tolerances required more difficult.
In reducing the costs of manufacture, it is desirable to maximize the number of holes etched in a dielectric layer in a single mask-and-etch cycle. However, etched holes are often required in portions of a dielectric layer having significantly different thicknesses. The simultaneous wet etching of holes through both thick and thin portions of a dielectric layer can be disastrous. Once the thin portion of the dielectric layer is etched through vertically, no further vertical etching occurs in that portion. However, wet etching must proceed for a significant time to allow for the completion of vertical etching through the thick portion of the dielectric layer. During this extra etching time, the thin portion of the dielectric layer will continue to be etched laterally, thereby increasing the size of the exposed portion of the underlying substrate. If the window in the mask overlying the dielectric layer cannot be made smaller, the excessive lateral etching may make it impossible to meet the required dimensional tolerances. The mask window may already be of the smallest size possible within the limits of existing technology, making the simultaneous etching of both the thick and thin portions of the dielectric layer impractical.
It is therefore desirable to create an etching process capable of eliminating a mask-and-etch cycle in integrated circuit manufacture by simultaneously etching both the thick and thin portions of a dielectric layer on a semiconductor substrate with a minimum of lateral etching.